1. Field of the Invention
The present invention relates to a semiconductor light-emitting element, and in particular, relates to a GaN based semiconductor light-emitting element.
2. Description of the Related Art
In recent years, a semiconductor light-emitting element using a GaN based semiconductor is known as a blue semiconductor light-emitting element. A white LED combining a blue semiconductor light-emitting element and a yellow luminous body has been in increasing demand to be used for LCD backlights of mobile phones and the like. Since the white LED has characteristics in lower power consumption and long life, it is expected to be used next as a light source replacing fluorescent lamps, incandescent lamps and the like.
A conventional GaN based semiconductor light-emitting element has a structure in which a GaN buffer layer, an n-type GaN layer, an emission layer, and a p-type GaN layer are sequentially grown as crystals on a sapphire substrate. However, such a conventional structure has a problem that a difference between an index of refraction of the p-type GaN layer and that of air or resin in contact with the p-type GaN layer is large and a total reflection angle at an interface between the p-type GaN layer and air or resin in contact with the p-type GaN layer becomes smaller. Therefore much light generated in the emission layer is totally reflected at the interface with air or resin in contact with the p-type GaN layer, thereby making light extraction inefficient.
If, for example, a semiconductor light-emitting element is emitting light in the air, the index of refraction of GaN is about 2.5 when the wavelength of light is 450 nm and therefore, the total reflection angle at the interface between the p-type GaN layer and air is small as much as about 24°. Light emitted from the emission layer and incident on the interface between the p-type GaN layer and air at an angle greater than the total reflection angle is totally reflected at the interface between the p-type GaN layer and air and thus cannot be extracted from the semiconductor light-emitting element.
A method of periodically forming unevenness on the p-type GaN layer at intervals comparable to the wavelength of luminous light has been proposed to handle the problem (See, for example, Patent Document 1). With this structure, the traveling direction of light emitted from the emission layer changes due to a diffraction effect caused by unevenness formed periodically and light is diffracted to angles at which total reflection does not occur, thereby improving light extraction efficiency of a semiconductor light-emitting element.
In order to form such unevenness formed periodically on the p-type GaN layer, a resist is first formed on the p-type GaN layer grown as a crystal and then a resist pattern is formed by interference exposure method or the like. Then, portions not covered by the resist pattern are removed by dry etching such as the RIE process to form unevenness on the p-type GaN layer.
Patent Document 1: Japanese Patent Application (Laid Open) No. 2005-5679
However, if dry etching is performed to etch the p-type GaN layer, nitrogen holes are generated on the etched surface of the p-type GaN layer by plasma damage. Since such nitrogen holes act as a donor, a portion which has been converted to n-type will arise on the etched surface of the p-type GaN layer. If an n-type converted portion exists in part of the surface of the p-type GaN layer, the portion is + biased from the n side, though an n/p junction exists, and thus reverse-biased, resulting in an increased forward voltage of the semiconductor light-emitting element. In addition, an electric current injected into the emission layer of the n-type converted portion decreases, and further the current is not spread because resistance of the p-type GaN layer is high, resulting in a reduced effective luminous area of the semiconductor light-emitting element.
Thus, the n-type converted portion of the p-type GaN layer must be removed by wet etching or the like, but wet etching of GaN is difficult to perform and it is hard to remove the n-type converted portion completely and also manufacturing costs increase since the number of manufacturing processes increases.